Thin film transistors using thin film semiconductor materials

ABSTRACT

The present invention generally comprises TFTs having semiconductor material comprising oxygen, nitrogen, and one or more element selected from the group consisting of zinc, tin, gallium, cadmium, and indium as the active channel. The semiconductor material may be used in bottom gate TFTs, top gate TFTs, and other types of TFTs. The TFTs may be patterned by etching to create both the channel and the metal electrodes. Then, the source-drain electrodes may be defined by dry etching using the semiconductor material as an etch stop layer. The active layer carrier concentration, mobility, and interface with other layers of the TFT can be tuned to predetermined values. The tuning may be accomplished by changing the nitrogen containing gas to oxygen containing gas flow ratio, annealing and/or plasma treating the deposited semiconductor film, or changing the concentration of aluminum doping.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 12/184,914 (APPM/12581), filed Aug. 1, 2008, which claimsbenefit U.S. provisional patent application Ser. No. 60/953,683(APPM/012581L), filed Aug. 2, 2007. Each of the aforementioned relatedpatent applications is herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention generally relate to field effecttransistors (FETs) and thin film transistors (TFTs) having semiconductormaterials comprising oxygen, nitrogen, and one or more elements selectedfrom the group consisting of zinc, gallium, cadmium, indium, and tin.

2. Description of the Related Art

Current interest in TFT arrays is particularly high because thesedevices may be used in liquid crystal active matrix displays (LCDs) ofthe kind often employed for computer and television flat panels. TheLCDs may also contain light emitting diodes (LEDs) for back lighting.Further, organic light emitting diodes (OLEDs) have been used for activematrix displays, and these OLEDs require TFTs for addressing theactivity of the displays.

TFTs made with amorphous silicon have become the key components of theflat panel display industry. Unfortunately amorphous silicon does haveits limitations such as low mobility. The mobility required for OLEDs isat least 10 times higher than that achievable with amorphous silicon.The deposition temperature for amorphous silicon may be high, which cancause a Vth shift. A high current may be necessary for amorphoussilicon, which can lead to stability issues in OLEDs. Polysilicon, onthe other hand, has a higher mobility than amorphous silicon.Polysilicon is crystalline, which leads to non-uniform deposition. Dueto the limitations of amorphous silicon, OLED advancement has beendifficult.

In recent years, transparent TFTs have been created in which zinc oxidehas been used as the active channel layer. Zinc oxide is a compoundsemiconductor that can be grown as a crystalline material at relativelylow deposition temperatures on various substrates such as glass andplastic. Zinc oxide based TFTs may not degrade upon exposure to visiblelight. Therefore, a shield layer, as is necessary for silicon basedTFTs, is not present. Without the shield layer, the TFT remainstransparent. Zinc oxide, while having a mobility greater than amorphoussilicon, still has a low mobility.

Therefore, there is a need in the art for TFTs having transparent activechannels with high mobility.

SUMMARY OF THE INVENTION

The present invention generally comprises TFTs having semiconductormaterial comprising oxygen, nitrogen, and one or more element selectedfrom the group consisting of zinc, tin, gallium, cadmium, and indium asthe active channel. The semiconductor material may be used in bottomgate TFTs, top gate TFTs, and other types of TFTs. The TFTs may bepatterned by etching to create both the channel and the metalelectrodes. Then, the source-drain electrodes may be defined by dryetching using the semiconductor material as an etch stop layer. Theactive layer carrier concentration, mobility, and interface with otherlayers of the TFT can be tuned to predetermined values. The tuning maybe accomplished by changing the nitrogen containing gas to oxygencontaining gas flow ratio, annealing and/or plasma treating thedeposited semiconductor film, or changing the concentration of aluminumdoping.

In one embodiment, a TFT is disclosed. The transistor comprises asemiconductor layer comprising oxygen, nitrogen and one or more elementsselected from the group consisting of zinc, indium, tin, cadmium,gallium, and combinations thereof. In another embodiment, a TFTfabrication method is disclosed. The method comprises depositing asemiconductor layer over a substrate, the semiconductor layer comprisingoxygen, nitrogen and one or more elements selected from the groupconsisting of zinc, indium, tin, cadmium, gallium, and combinationsthereof.

In another embodiment, a TFT fabrication method is disclosed. The methodcomprises depositing a semiconductor layer over a substrate, thesemiconductor layer comprising oxygen, nitrogen and one or more elementsselected from the group consisting of an element having filled s and dorbitals, an element having a filled f orbital, and combinationsthereof. The method also includes depositing a source-drain electrodelayer over the active channel layer, first etching the source-drainelectrode layer and the active channel layer to create an activechannel, and second etching the source-drain electrode layer to definesource-drain electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a schematic cross sectional view of a PVD chamber according toone embodiment of the invention.

FIGS. 2A-2E are XRD graphs for films showing the formation of zinc andzinc oxide peaks as a function of oxygen gas flow.

FIGS. 3A-3F are XRD graphs for showing the formation of a semiconductorfilm according at various nitrogen gas flow rates according to oneembodiment of the invention.

FIGS. 4A-4G show a process sequence for forming a bottom gate TFTaccording to one embodiment of the invention.

FIG. 5 is a schematic cross sectional view of an etch stop TFT accordingto one embodiment of the invention.

FIG. 6 is a schematic cross sectional view of a top gate TFT accordingto one embodiment of the invention.

FIG. 7 is a schematic view of an active-matrix LCD according to oneembodiment of the invention.

FIG. 8 is a schematic view of an active-matrix OLED according to oneembodiment of the invention.

FIGS. 9A-9C show the Vth for various active channel lengths and widths.

FIGS. 10A-10C show a comparision of the Vth for active channels having acommon length and width.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements disclosed in oneembodiment may be beneficially utilized on other embodiments withoutspecific recitation.

DETAILED DESCRIPTION

The present invention generally comprises TFTs having semiconductormaterial comprising oxygen, nitrogen, and one or more element selectedfrom the group consisting of zinc, tin, gallium, cadmium, and indium asthe active channel. The semiconductor material may be used in bottomgate TFTs, top gate TFTs, and other types of TFTs. The TFTs may bepatterned by etching to create both the channel and the metalelectrodes. Then, the source-drain electrodes may be defined by dryetching using the semiconductor material as an etch stop layer. Theactive layer carrier concentration, mobility, and interface with otherlayers of the TFT can be tuned to predetermined values. The tuning maybe accomplished by changing the nitrogen containing gas to oxygencontaining gas flow ratio, annealing and/or plasma treating thedeposited semiconductor film, or changing the concentration of aluminumdoping.

The semiconductor film comprising nitrogen, oxygen, and one or moreelements selected from zinc, indium, gallium, cadmium, and tin may bedeposited by reactive sputtering. The reactive sputtering method isillustratively described and may be practiced in a PVD chamber forprocessing large area substrates, such as a 4300 PVD chamber, availablefrom AKT America, Inc., a subsidiary of Applied Materials, Inc., SantaClara, Calif. However, because the semiconductor film produced accordingto the method may be determined by the film structure and composition,it should be understood that the reactive sputtering method may haveutility in other system configurations, including those systemsconfigured to process large area round substrates and those systemsproduced by other manufacturers, including roll-to-roll processplatforms. It is to be understood that while the invention isillustratively described below as deposited by PVD, other methodsincluding chemical vapor deposition (CVD), atomic layer deposition(ALD), or spin-on processes may be utilized to deposit the inventivefilms.

FIG. 1 is a cross-sectional schematic view of a PVD chamber 100according to one embodiment of the invention. The chamber 100 may beevacuated by a vacuum pump 114. Within the chamber 100, a substrate 102may be disposed opposite a target 104. The substrate may be disposed ona susceptor 106 within the chamber 100. The susceptor 106 may beelevated and lowered as shown by arrows “A” by an actuator 112. Thesusceptor 106 may be elevated to raise the substrate 102 to a processingposition and lowered so that the substrate 102 may be removed from thechamber 100. Lift pins 108 elevate the substrate 102 above the susceptor106 when the susceptor 106 is in the lowered position. Grounding straps110 may ground the susceptor 106 during processing. The susceptor 106may be raised during processing to aid in uniform deposition.

The target 104 may comprise one or more targets 104. In one embodiment,the target 104 may comprise a large area sputtering target 104. Inanother embodiment, the target 104 may comprise a plurality of tiles. Inyet another embodiment, the target 104 may comprise a plurality oftarget strips. In still another embodiment, the target 104 may compriseone or more cylindrical, rotary targets. The target 104 may be bonded toa backing plate 116 by a bonding layer (not shown). One or moremagnetrons 118 may be disposed behind the backing plate 116. Themagnetrons 118 may scan across the backing plate 116 in a linearmovement or in a two dimensional path. The walls of the chamber may beshielded from deposition by a dark space shield 120 and a chamber shield122.

To help provide uniform sputtering deposition across a substrate 102, ananode 124 may be placed between the target 104 and the substrate 102. Inone embodiment, the anode 124 may be bead blasted stainless steel coatedwith arc sprayed aluminum. In one embodiment, one end of the anode 124may be mounted to the chamber wall by a bracket 130. The anode 124provides a charge in opposition to the target 104 so that charged ionswill be attracted thereto rather than to the chamber walls which aretypically at ground potential. By providing the anode 124 between thetarget 104 and the substrate 102, the plasma may be more uniform, whichmay aid in the deposition. To reduce flaking, a cooling fluid may beprovided through the one or more anodes 124. By reducing the amount ofexpansion and contraction of the anodes 124, flaking of material fromthe anodes 124 may be reduced. For smaller substrates and hence, smallerprocessing chambers, the anodes 124 spanning the processing space maynot be necessary as the chamber walls may be sufficient to provide apath to ground and a uniform plasma distribution.

For reactive sputtering, it may be beneficial to provide a reactive gasinto the chamber 100. One or more gas introduction tubes 126 may alsospan the distance across the chamber 100 between the target 104 and thesubstrate 102. For smaller substrates and hence, smaller chambers, thegas introduction tubes 126 spanning the processing space may not benecessary as an even gas distribution may be possible throughconventional gas introduction means. The gas introduction tubes 126 mayintroduce sputtering gases from a gas panel 132. The gas introductiontubes 126 may be coupled with the anodes 124 by one or more couplings128. The coupling 128 may be made of thermally conductive material topermit the gas introduction tubes 126 to be conductively cooled.Additionally, the coupling 128 may be electrically conductive as well sothat the gas introduction tubes 126 are grounded and function as anodes.

The reactive sputtering process may comprise disposing a metallicsputtering target opposite a substrate in a sputtering chamber. Themetallic sputtering target may substantially comprise one or moreelements selected from the group consisting of zinc, gallium, indium,tin, and cadmium. In one embodiment, the sputtering target may compriseone or more elements having a filled s orbital and a filled d orbital.In another embodiment, the sputtering target may comprise one or moreelements having a filled f orbital. In another embodiment, thesputtering target may comprise one or more divalent elements. In anotherembodiment, the sputtering target may comprise one or more trivalentelements. In still another embodiment, the sputtering target maycomprise one or more tetravalent elements.

The sputtering target may also comprise a dopant. Suitable dopants thatmay be used include Al, Sn, Ga, Ca, Si, Ti, Cu, Ge, In, Ni, Mn, Cr, V,Mg, Si_(x)N_(y), Al_(x)O_(y), and SiC. In one embodiment, the dopantcomprises aluminum. In another embodiment, the dopant comprises tin. Thesubstrate, on the other hand, may comprise plastic, paper, polymer,glass, stainless steel, and combinations thereof. When the substrate isplastic, the reactive sputtering may occur at temperatures below about180 degrees Celsius. Examples of semiconductor films that may bedeposited include ZnO_(x)N_(y):Al, ZnO_(x)N_(y):Sn, SnO_(x)N_(y):Al,InO_(x)N_(y):Al, InO_(x)N_(y):Sn, CdO_(x)N_(y):Al, CdO_(x)N_(y):Sn,GaO_(x)N_(y):Al, GaO_(x)N_(y):Sn, ZnSnO_(x)N_(y):Al ZnInO_(x)N_(y):Al,ZnInO_(x)N_(y):Sn, ZnCdO_(x)N_(y):Al, ZnCdO_(x)N_(y):Sn,ZnGaO_(x)N_(y):Al, ZnGaO_(x)N_(y):Sn, SnInO_(x)N_(y):Al,SnCdO_(x)N_(y):Al, SnGaO_(x)N_(y):Al, InCdO_(x)N_(y):Al,InCdO_(x)N_(y):Sn, InGaO_(x)N_(y):Al, InGaO_(x)N_(y):Sn,CdGaO_(x)N_(y):Al, CdGaO_(x)N_(y):Sn, ZnSnInO_(x)N_(y):Al,ZnSnCdO_(x)N_(y):Al, ZnSnGaO_(x)N_(y):Al, ZnInCdO_(x)N_(y):Al,ZnInCdO_(x)N_(y):Sn, ZnInGaO_(x)N_(y):Al, ZnInGaO_(x)N_(y):Sn,ZnCdGaO_(x)N_(y):Al, ZnCdGaO_(x)N_(y):Sn, SnInCdO_(x)N_(y):Al,SnInGaO_(x)N_(y):Al, SnCdGaO_(x)N_(y):Al, InCdGaO_(x)N_(y):Al,InCdGaO_(x)N_(y):Sn, ZnSnInCdO_(x)N_(y):Al, ZnSnInGaO_(x)N_(y):Al,ZnInCdGaO_(x)N_(y):Al, ZnInCdGaO_(x)N_(y):Sn, and SnInCdGaO_(x)N_(y):Al.

During the sputtering process, argon, a nitrogen containing gas, and anoxygen containing gas may be provided to the chamber for reactivesputtering the metallic target. Additional additives such as B₂H₆, CO₂,CO, CH₄, and combinations thereof may also be provided to the chamberduring the sputtering. In one embodiment, the nitrogen containing gascomprises N₂. In another embodiment, the nitrogen containing gascomprises N₂O, NH₃, or combinations thereof. In one embodiment, theoxygen containing gas comprises O₂. In another embodiment, the oxygencontaining gas comprises N₂O. The nitrogen of the nitrogen containinggas and the oxygen of the oxygen containing gas react with the metalfrom the sputtering target to form a semiconductor material comprisingmetal, oxygen, nitrogen, and optionally a dopant on the substrate. Inone embodiment, the nitrogen containing gas and the oxygen containinggas are separate gases. In another embodiment, the nitrogen containinggas and the oxygen containing gas comprise the same gas.

The film deposited is a semiconductor film. Examples of semiconductorfilms that may be deposited include ZnO_(x)N_(y), SnO_(x)N_(y),InO_(x)N_(y), CdO_(x)N_(y), GaO_(x)N_(y), ZnSnO_(x)N_(y),ZnInO_(x)N_(y), ZnCdO_(x)N_(y), ZnGaO_(x)N_(y), SnInO_(x)N_(y),SnCdO_(x)N_(y), SnGaO_(x)N_(y), InCdO_(x)N_(y), InGaO_(x)N_(y),CdGaO_(x)N_(y), ZnSnInO_(x)N_(y), ZnSnCdO_(x)N_(y), ZnSnGaO_(x)N_(y),ZnInCdO_(x)N_(y), ZnInGaO_(x)N_(y), ZnCdGaO_(x)N_(y), SnInCdO_(x)N_(y),SnInGaO_(x)N_(y), SnCdGaO_(x)N_(y), InCdGaO_(x)N_(y),ZnSnInCdO_(x)N_(y), ZnSnInGaO_(x)N_(y), ZnInCdGaO_(x)N_(y), andSnInCdGaO_(x)N_(y). Each of the aforementioned semiconductor films maybe doped by a dopant.

The semiconductor film may comprise an oxynitride compound. In oneembodiment, the semiconductor film comprises both a metal oxynitridecompound as well as a metal nitride compound. In another embodiment, thesemiconductor film may comprise a metal oxynitride compound, a metalnitride compound, and a metal oxide compound. In still anotherembodiment, the semiconductor film may comprise a metal oxynitridecompound and a metal oxide compound. In another embodiment, thesemiconductor film may comprise a metal nitride compound and a metaloxide compound.

The ratio of the nitrogen containing gas to the oxygen containing gasmay affect the mobility, carrier concentration, and resistivity of thesemiconductor film. Table I shows the effect of the nitrogen flow rateon the mobility, resistivity, and carrier concentration for a tin targetsputtered in an atmosphere of argon and nitrogen gas. Generally, Table Ishows that when the nitrogen flow rate increases, the mobility alsoincreases. The argon and oxygen flow rates may remain the same. In TableI, the argon flow rate is 60 sccm and the oxygen flow rate is 5 sccm.The higher substrate temperature also provides an increase in mobility.The carrier concentration is weakly correlated with the mobility. Thedeposited film is an n-type semiconductor material which may function asan electron carrier and hence, the carrier concentration is shown as anegative number.

TABLE I Carrier N₂ Flow Concen- Rate Temperature Mobility trationResistivity (sccm) (Celsius) (cm²/V-s) (#/cc) (ohm-cm) 0 150 0.3−1.00E+23 0.0005 50 150 0.2 −6.00E+20 0.12 100 150 1.1 −2.00E+15 7000150 150 1.1 −5.00E+19 0.12 200 150 3.7 −8.00E+19 0.05 0 250 0.4−3.00E+18 10 50 250 0.2 −3.00E+21 0.09 100 250 0.4 −9.00E+17 90 150 2501.8 −3.00E+18 3 200 250 7.1 9.00E+19 0.01

The oxygen containing gas also affects the mobility, carrierconcentration, and resistivity of the semiconductor film. Table II showsthe effect of the oxygen flow rate on the mobility, resistivity, andcarrier concentration for a tin target sputtered in an atmosphere ofargon, nitrogen gas, and oxygen gas. The argon flow rate may remain thesame. In Table II, the argon flow rate is 60 sccm. Generally, Table IIshows that for high nitrogen gas to oxygen gas ratios, the mobility maybe higher than the mobility for amorphous silicon. Additionally, thehigher the ratio of nitrogen to oxygen, the lower the carrierconcentration. At a 200 sccm nitrogen flow rate, the mobility increasesas the oxygen flow rate increase, but then decreases at higher oxygenflow rates. In one embodiment, the mobility may be between about 4cm²/V-s and about 10 cm²/V-s at a temperature of 150 degrees Celsius.The increase in mobility is not correlated to the carrier concentration.Thus, the mobility improvement may be a result of less scattering of thecarrier. The mobility may be very low if no nitrogen additives are used.In such a scenario, the carrier concentration drops significantly as theoxygen gas flow increases. The higher the substrate temperature for atin target, the better the mobility. In one embodiment, the pressure maybe between about 5 mTorr to about 20 mTorr.

TABLE II Carrier O₂ Flow N₂ Flow Tempera- Concen- Resis- Rate Rate tureMobility tration tivity (sccm) (sccm) (Celsius) (cm²/V-s) (#/cc)(ohm-cm) 5 0 150 0.1 −2.00E+23 0.0005 10 0 150 0.2 −2.00E+18 40 0 200150 1.2 −3.00E+19 0.03 5 200 150 4.2 −9.00E+19 0.04 10 200 150 3.5−1.00E+20 0.05 0 200 250 0.2 −3.00E+19 10 5 200 250 7.0 −9.00E+19 0.0110 200 250 9.5 −9.00E+19 0.008

The amount of dopant may also affect the mobility of the deposited film.However, the mobility will still generally increase with an increase ofnitrogen gas flow whether the target is doped or not. Table III showsthe effect of dopant upon the mobility, carrier concentration, andresistivity. The dopant is shown in weight percentage. The argon flowrate may be the same for each deposited film. In Table III, the argonflow rate is 120 sccm. The carrier concentration when utilizing a dopantmay be lower than in the scenario where no dopant is used. Thus, thedopant may be used to tune the carrier concentration.

TABLE III O₂ N₂ Per- Mobil- Carrier Resis- Flow Flow cent Tempera- ityConcen- tivity Rate Rate Dopant ture (cm²/ tration (ohm- (sccm) (sccm)(%) (Celsius) V-s) (#/cc) cm) 10 100 1 50 1 −1.00E+21 0.009 10 200 1 5014 −4.00E+19 0.02 10 300 1 50 31 −9.00E+18 0.04 10 400 1 50 34 −5.00E+180.07 10 500 1 50 34 −4.00E+18 0.09 20 100 0 50 2 −1.00E+21 0.008 20 2000 50 14 −8.00E+19 0.009 20 300 0 50 29 −2.00E+19 0.02 20 400 0 50 42−1.00E+19 0.03 20 500 0 50 45 −8.00E+18 0.04 20 100 1 50 13 −5.00E+190.03 20 200 1 50 27 −3.00E+18 0.01 20 300 1 50 29 −2.00E+18 0.01 20 4001 50 29 −2.00E+18 0.01 20 500 1 50 32 −1.00E+18 0.03

Table IV discloses the effect of oxygen gas flow on the mobility,carrier concentration, and resistivity of the semiconductor film.Generally, under a fixed nitrogen gas flow, the mobility of the filmwill increase as the oxygen flow increases, but drop with a furtherincrease in oxygen flow rate. The argon flow rate may be the same foreach deposited film. In Table IV, the argon flow rate is 120 sccm. Inone embodiment, the mobility of the film will decrease once the nitrogencontaining gas to oxygen containing gas ratio is less than about 10:1.The increase in mobility does not relate to an increase in carrierconcentration as the oxygen flow rate increases. When a dopant is used,the mobility and carrier concentration may be lowered. Thus, the carrierconcentration and mobility may be tuned with the amount of dopantpresent.

TABLE IV O₂ N₂ Per- Mobil- Carrier Resis- Flow Flow cent Tempera- ityConcen- tivity Rate Rate Dopant ture (cm²/ tration (ohm- (sccm) (sccm)(%) (Celsius) V-s) (#/cc) cm) 0 300 1 50 0 −3.00E+21 0.03 10 300 1 50 31−9.00E+18 0.04 30 300 1 50 23 −9.00E+17 0.7 0 500 1 50 3 −2.00E+19 0.0210 500 1 50 33 −5.00E+18 0.009 20 500 1 50 32 −1.00E+18 0.2 30 500 1 5025 −5.00E+17 0.9 40 500 1 50 10 −1.00E+16 10 10 300 0 50 4 −2.00E+200.009 20 300 0 50 30 −2.00E+19 0.01 30 300 0 50 43 −1.00E+19 0.02 40 3000 50 9 −1.00E+16 80 10 500 0 50 23 −9.00E+18 0.05 20 500 0 50 46−8.00E+19 0.04 30 500 0 50 47 −7.00E+18 0.05 40 500 0 50 34 −9.00E+170.3 50 500 0 50 15 −4.00E+16 20

Table V shows the affect of the power density applied on the mobility,carrier concentration, and resistivity of the semiconductor film.Generally, the power density does not greatly affect the mobility, butthe higher the power density, the higher the carrier concentration andresistivity. In one embodiment, the power density applied to thesputtering target may be between about 0.3 W/cm² and about 1.0 W/cm².

TABLE V O₂ Flow N₂ Flow Percent Power Carrier Rate Rate Dopant appliedTemperature Mobility Concentration Resistivity (sccm) (sccm) (%) (W/cm²)(Celsius) (cm²/V-s) (#/cc) (ohm-cm) 0 500 2.8 0.47 50 13 −8.00E+17 1 10500 2.8 0.47 50 19 −1.00E+18 0.03 20 500 2.8 0.47 50 30 −8.00E+17 0.0830 500 2.8 0.47 50 22 −5.00E+15 95 0 500 2.8 0.7 50 3 −1.00E+18 0.8 10500 2.8 0.7 50 30 −1.00E+18 0.3 20 500 2.8 0.7 50 29 −2.00E+18 0.1 30500 2.8 0.7 50 20 −8.00E+17 0.9 40 500 2.8 0.7 50 4 −2.00E+16 200

Table VI shows the effects of utilizing N₂O as the oxygen containing gasin depositing the semiconductor film. The N₂O gas is effective as anoxygen containing gas in raising the mobility of the semiconductor filmand producing a reasonably low carrier concentration.

TABLE VI N₂O N₂ Flow Flow Power Tempera- Mobil- Carrier Rate Rateapplied ture ity Concentration (sccm) (sccm) (W/cm²) (Celsius) (cm²/V-s)(#/cc) 10 500 0.35 50 18 −9.00E+18 20 500 0.35 50 42 −9.00E+18 30 5000.35 50 58 −8.00E+18 40 500 0.35 50 57 −7.00E+18 50 500 0.35 50 52−5.00E+18 60 500 0.35 50 40 −2.00E+18 70 500 0.35 50 25 −2.00E+17 10 5000.35 180 95 −1.00E+19 20 500 0.35 180 110 −9.00E+18 30 500 0.35 180 75−5.00E+17 40 500 0.35 180 65 −9.00E+17 50 500 0.35 180 55 −5.00E+18 60500 0.35 180 40 −2.00E+18 70 500 0.35 180 25 −6.00E+17

Table VII shows the chemical analysis for a semiconductor film thatcomprises tin, oxygen, and nitrogen and shows the effect of oxygencontaining gas upon the film using X-ray photoelectron spectroscopy(XPS). Film 1 was deposited by sputtering a tin target for 360 secondswhile a DC bias of 400 W was applied to the sputtering target. Argon wasintroduced to the processing chamber at a flow rate of 60 sccm, nitrogenwas introduced at a flow rate of 200 sccm, and oxygen was introduced ata flow rate of 5 sccm. The deposition occurred at a temperature of 250degrees Celsius. Film 1 had a carbon content of Film 1 was 22.5 atomicpercent, a nitrogen content of 19.4 atomic percent, an oxygen content of29.4 atomic percent, a fluorine content of 0.7 atomic percent, and a tincontent of 28.1 atomic percent. Most, if not all, of the carbon couldarise from adventitious carbon (i.e., carbon compounds adsorbed onto thesurface of any sample exposed to the atmosphere). Film 2 was depositedby sputtering a tin target for 360 seconds while a DC bias of 400 W wasapplied to the sputtering target. Argon was introduced to the processingchamber at a flow rate of 60 sccm, nitrogen was introduced at a flowrate of 200 sccm, and oxygen was introduced at a flow rate of 20 sccm.The deposition occurred at a temperature of 250 degrees Celsius. Film 2had a carbon content of 17.3 atomic percent, a nitrogen content of 4.5atomic percent, an oxygen content of 49.9 atomic percent, a fluorinecontent of 0.6 percent, and a tin content of 27.7 atomic percent. Most,if not all, of the carbon could arise from adventitious carbon (i.e.,carbon compounds adsorbed onto the surface of any sample exposed to theatmosphere). As shown in Table VII, as the oxygen flow rate (and hence,the ratio of oxygen to nitrogen) increases, the oxynitride contentincreases as well as does the tin oxide content. However, the tinnitride content and silicon oxynitride content is reduced. In Table VII,R equals oxygen or nitrogen.

TABLE VII Beam Energy Film 1 Film 2 (eV) Carbon Chemical State C—C,CH_(x) 76.0 73.7 285.0 C—R 12.6 12.0 286.4 O═C 2.8 4.4 287.9 O═C—R 8.610.0 289.1 Nitrogen Chemical State Nitride 79.0 56.2 396.9 Oxynitride19.5 22.7 397.7 Organic N 1.3 10.4 399.1 NH⁴⁺ 0.0 10.7 402.8 TinChemical State Sn 1.4 2.1 484.8 SnO₂ 71.9 84.4 486.4 SnN_(x), 26.7 13.4487.3 SnON Oxygen Chemical State SnO_(x) 51.7 69.8 530.7 Organic 48.330.2 532.1 O

Table VIII shows the results for several semiconductor films that weredeposited by sputtering. The semiconductor films comprised zinc, tin,oxygen, and nitrogen. The semiconductor films were sputter depositedfrom a sputtering target having a zinc content of 70 atomic percent anda tin content of 30 atomic percent. The deposition occurred at atemperature of 250 degrees Celsius with a power of 400 W applied to thesputtering target. The deposition occurred for 360 seconds under anargon flow rate of 60 sccm and an oxygen flow rate of 20 sccm. The datashows that the mobility of the semiconductor film increases as thenitrogen flow rate (and hence, the ratio of nitrogen gas to oxygen gas)increases.

TABLE VIII SEM N₂ Thickness Mobility Film (sccm) (micrometers) Rs(cm²/V-s) Ns N 1 0.0 1.82 3.65E+05 0.0155 −1.10E+15 −6.06E+18 2 100.00.18 2.88E+07 1.1200 1.93E+11 1.05E+16 3 150.0 0.18 9.69E+03 4.7000−1.37E+14 −7.44E+18 4 200.0 0.17 1.33E+03 14.0000 −3.34E+14 −1.93E+19

Zinc, Oxygen, and Nitrogen Compounds

In order to determine the desired oxygen flow rate for forming thesemiconductor film comprising zinc, oxygen, and nitrogen, the amount ofoxygen may be selected so that the amount of oxygen is not sufficient tocompletely oxidize the zinc to form zinc oxide. If the amount of oxygencontaining gas supplied is too high, the mobility of the film may not besufficient because the film may be too oxidized. The amount of oxidationof zinc may affect the transmittance. For example, completely oxidizedzinc may have a transmittance of greater than about 80 percent. Onemanner of determining the desired oxygen flow is to run a reactivesputtering process using argon and oxygen gases without using nitrogengas. Experiments may be performed at different oxygen flow rates and theoptical transmittance in the visible wavelength may be measured. Thedesired oxygen flow may be just before the film has a maximumtransparency that may be achieved. Table IX shows the opticaltransmittance for zinc oxide reactively sputter deposited at variousoxygen flow rates. In one embodiment, the maximum preferredtransmittance may be 80 percent. In other embodiments, the maximumtransmittance may not be 80 percent if the glass absorption or lightinterference is included. The experiments may be useful when usingdifferent DC target power, different substrate temperature, or evendifferent oxygen containing gases such as N₂O.

TABLE IX Oxygen Flow Rate Transmittance (sccm/m³) (%) 0 <5 50 <5 100 <5125 82 150 85 200 90

Another method to determine the desired oxygen gas flow is to performthe reactive sputtering to form zinc oxide under the condition ofproviding no nitrogen or a low amount of nitrogen as discussed above andthen measure the sheet resistance. An oxygen flow rate that produces asheet resistance between about 100 ohm/sq and 1.0×10⁷ ohm/sq may be thedesired oxygen flow rate.

Yet another manner for determining the desired oxygen flow rate is totake an XRD film structure measurement. FIGS. 2A-2E are XRD graphs forfilms showing the formation of zinc and zinc oxide peaks as a functionof oxygen gas flow. Each of the films shown in FIGS. 2A-2E weredeposited at an argon flow rate of 600 sccm/m³ and 1,000 W and variousoxygen flow rates.

FIG. 2A shows an XRD graph of a film formed when no oxygen gas isprovided during the sputtering. Several zinc peaks were produced havingvarious intensities. A zinc (002) peak is shown for 2 theta (i.e., theangle between the incident x-ray and the detector of the diffractometer)between about 35.5 and 37 with an intensity of about 625 counts. A zinc(100) peak is shown between about 38 and 40 with an intensity of about450 counts. A zinc (101) peak is shown between about 42.5 and 44 with anintensity of about 1050 counts. A zinc (102) peak is shown between about53 and 55 with an intensity of about 325 counts. A zinc (103) peak isshown between about 69.5 and 70 with an intensity of about 300. A zincpeak (110) peak is shown between about 70 and 71 with an intensity ofabout 275 counts. The ratio of peak heights for the zinc (002):zinc(100):zinc (101):zinc (102):zinc (103):zinc (110) is about2.27:1.64:3.82:1.182:1.091:1. All peaks are marked using theInternational Center for Diffraction Data (ICDD) PDF2 database (rev.2004) for phase identification.

When oxygen gas is provided at a flow rate of 50 sccm/m³, the zinc peaksdiminish in intensity as shown in FIG. 2B. The zinc (002) peakdiminishes to about 500 counts. The zinc (100) peak diminishes to about375 counts. The zinc (101) peak diminishes to about 750 counts. The zinc(102) peak diminishes to about 250 counts. The zinc (110) peakdiminishes to about 225 counts, and the zinc (103) peak is not present.The ratio of the peak heights for zinc (002):zinc (100):zinc (101):zinc(102):zinc (110) is about 2.22:1.67:3.33:1.11:1.

When the oxygen gas is provided at a flow rate of 100 sccm/m³, all ofthe zinc peaks disappear except the zinc (101) peak which has diminishedto about 375 counts as shown in FIG. 2C. When the oxygen gas is providedat 150 sccm/m³, the zinc peaks are completely gone, but a zinc oxide(002) peak appears between about 33.5 and 35 with an intensity of about950 counts as shown in FIG. 2D. When the oxygen flow rate is increasedto 200 sccm/m³, the zinc oxide (002) peak increases in intensity toabout 1,000 counts as shown in FIG. 2E.

The amount of oxygen supplied, according to the XRD data, should be lessthan about 150 sccm/m³ because at 150 sccm/m³ a strong zinc oxide peakappears. It is to be understood that the flow rate of oxygen isproportional to the chamber size. Thus, for as the size of the chamberincreases, the oxygen flow rate may also increase. Similarly, as thesize of the chamber is reduced, the oxygen flow rate may decrease.

To determine the desired nitrogen flow rate, XRD film structuremeasurements may be taken. FIGS. 3A-3F are XRD graphs for showing theformation of a semiconductor film according at various nitrogen gas flowrates according to one embodiment of the invention. Each of the filmsshown in FIGS. 3A-3F were deposited at an argon flow rate of 600sccm/m³, 2,000 W, an oxygen flow rate of 100 sccm/m³, and variousnitrogen flow rates.

FIG. 3A shows an XRD graph of a film deposited with no nitrogen. Thegraph reveals several strong peaks including a peak between about 35 andabout 37 of zinc oxide (101) and zinc (002) having an intensity of about575 counts, a peak between about 38 and 40 of zinc (100) having anintensity of about 380 counts, and a peak between about 42.5 and 44 ofzinc (101) having an intensity of about 700 counts. Smaller peaks ofzinc oxide (002) between about 35.5 and 37 with an intensity of about390 counts, zinc (102) between about 53 and 55 with an intensity ofabout 275 counts, zinc (103) between about 69.5 and 70 with an intensityof about 225 counts, and a peak of zinc (110) between about 70 and 71with an intensity of about 225 counts are also present. The ratio of thepeak heights for zinc oxide (101):zinc (002):zinc (100):zinc (101):zincoxide (002):zinc (102):zinc (103):zinc (110) is about2.55:2.55:1.24:3.11:1.73:1.22:1:1.

When nitrogen is provided during the reactive sputtering at a flow rateof 300 sccm/m³, the zinc and zinc oxide peaks have significantlydiminished to the point where zinc oxide may no longer be present asshown in FIG. 3B. When the nitrogen flow rate is increased to 500sccm/m³, all of the zinc and zinc oxide peaks have diminished and thefilm has an amorphous structure as shown in FIG. 3C.

When the nitrogen flow rate is increased to 1,000 sccm/m³, two new peaksappear as shown in FIG. 3D. A first peak of Zn₃N₂ ₍222) has formedbetween about 31 and 33 with an intensity of about 2050 counts. A secondpeak of Zn₃N₂ (411) has formed between about 35 and 42 with an intensityof about 1850 counts. The ratio of peak heights for Zn₃N₂ (222):Zn₃N₂(411) is about 1.11:1. When the nitrogen gas flow rate is increased to1,250 sccm/m³, the Zn₃N₂ (222) peak intensifies to about 2500 counts andthe Zn₃N₂ (411) peak intensifies to about 2600 counts as shown in FIG.3E. The ratio of peak heights for Zn₃N₂ (222):Zn₃N₂ (411) is about0.96:1. When the nitrogen flow rate is increased to 2,500 sccm/m³, theZn₃N₂ (222) peak and the Zn₃N₂ (411) weaken to about 2350 and 2050respectively, but a new peak of Zn₃N₂ (400) develops between about 36and 37.5 with an intensity of about 1700 counts as shown in FIG. 3F. Theratio of peak heights for Zn₃N₂ (222):Zn₃N₂ (411):Zn₃N₂ (400) is about1.38:1.21:1.

The amount of nitrogen supplied, according to the XRD data, should begreater than about 300 sccm/m³ because at 300 sccm/m³ the zinc oxidepeaks diminish significantly such that essentially no zinc oxide ispresent in the film. It is to be understood that the flow rate ofnitrogen is proportional to the chamber size. Thus, as the size of thechamber increases, the nitrogen flow rate may also increase. Similarly,as the size of the chamber is reduced, the nitrogen flow rate maydecrease.

Therefore, combining the oxygen flow rates from above and the nitrogenflow rates from above, the new semiconductor film discussed herein maybe deposited under a nitrogen to oxygen flow rate ratio of greater thanabout 2:1. In one embodiment, the flow ratio of nitrogen to oxygen maybe 10:1 to about 50:1. In still another embodiment, the flow ratio ofnitrogen to oxygen may be 20:1.

To produce the semiconductor material, the flow rate of the nitrogencontaining gas may be much greater than the flow rate of the oxygencontaining gas as discussed above. The deposited semiconductor materialmay have a mobility greater than amorphous silicon. Table X shows themobility as a function of nitrogen gas flow rate according to oneembodiment of the invention.

TABLE X Nitrogen Oxygen Flow Rate Flow Rate Mobility (sccm/m³) (sccm/m³)(cm²/V-s) 500 50 1 100 13.5 250 5 1,000 50 14 100 27 1,500 0 <1 25 8 5031 150 23.5 200 1 250 2 2,000 0 1 50 34 100 29 2,500 0 2.5 25 15 50 33.5100 33 150 25 200 10 250 12

Films deposited under conditions of 0 sccm oxygen had mobility of lessthan 5 cm²/V-s for all flow rates of nitrogen gas. Films deposited underconditions of 25 sccm/m³ oxygen had a mobility of about 8 cm²/V-s for anitrogen flow rate of 1,500 sccm/m³ and about 15 cm²/V-s for a nitrogenflow rate of 2,500 sccm/m³. Films deposited under conditions of 200sccm/m³ oxygen had a mobility of about 1 cm²/V-s for a nitrogen flowrate of 1,500 sccm/m³ and a mobility of about 10 cm²/V-s for a nitrogenflow rate of 2,500 sccm/m³. Films deposited under conditions of 250sccm/m³ oxygen has a mobility of about 5 cm²/V-s for a nitrogen flowrate of 500 sccm/m³, about 2 cm²/V-s for a nitrogen flow rate of 1,500sccm/m³, and about 12 cm²/V-s for a nitrogen flow rate of 2,500 sccm/m³.

For films deposited with an oxygen flow rate of between 50 sccm/m³ and150 sccm/m³, the mobility of the films was markedly increased over thefilms deposited at oxygen flow rates of 25 sccm/m³ and below and filmsdeposited at oxygen flow rates of 200 sccm/m³ and above. Additionally,the films deposited with an oxygen flow rate of between 50 sccm/m³ and150 sccm/m³ have mobilities far greater than amorphous silicon. Atnitrogen flow rates of between 1,000 sccm/m³ and 2,500 sccm/m³, themobility of the films were, in most cases, higher than 22 cm²/V-s. Whencompared to amorphous silicon, which has a mobility of about 1 cm²/V-s,the semiconductor films containing zinc, oxygen, and nitrogen have asignificant improvement in mobility. Hence, nitrogen to oxygen gas flowratios of about 10:1 to about 50:1 may produce semiconductor filmshaving mobility greater than 20 times the mobility of amorphous siliconand 2 times the mobility of polysilicon. It is to be understood thatwhile the table shows specific flow rates of nitrogen gas and oxygengas, the flow rates of the oxygen gas and nitrogen gas are relative tothe chamber size and thus, are scalable to account for different chambersizes.

Table XI shows the sheet resistance, carrier concentration, andresistivity as a function of nitrogen gas flow rate according to oneembodiment of the invention. For flow ratios of nitrogen gas to oxygengas between about 10:1 to about 50:1, the sheet resistance of thesemiconductor layer comprising zinc, oxygen, and nitrogen may be betweenabout 100 ohm/sq and about 10,000 ohm/sq. With an increase in bothnitrogen flow rate and oxygen flow rate, the electron carrierconcentration lowers. Consequently, the resistivity increases.

TABLE XI Carrier Nitrogen Oxygen Sheet Concen- Flow Rate Flow RateResistance tration Resistivity (sccm/m³) (sccm/m³) (ohm-cm) (#/cc)(ohm-cm) 500 50 400 1.00E+21 0.009 100 800 5.00E+19 0.012 1,000 50 7505.00E+19 0.012 100 5000 4.00E+18 0.1 1,500 0 600 4.00E+21 0.014 50 9509.00E+18 0.014 2,000 0 1000 9.00E+20 0.014 50 2000 5.00E+18 0.017 1009000 2.00E+18 0.1 2,500 0 6000 2.00E+19 0.11 50 5000 4.00E+18 0.09 1009000 1.50E+18 0.12

Annealing may also significantly raise the mobility of the semiconductorfilm containing zinc, oxygen, and nitrogen. Table XII shows the mobilityas a function of nitrogen gas flow rate after annealing according to oneembodiment of the invention. After annealing, the mobility may begreater than 50 cm²/V-s. In one embodiment, the mobility may beincreased to greater than 90 cm²/V-s by annealing. The annealing mayoccur for about five minutes in a nitrogen atmosphere at a temperatureof about 400 degrees Celsius.

TABLE XII Nitrogen Oxygen Flow Rate Flow Rate Mobility (sccm/m³)(sccm/m³) (cm²/V-s) 500 0 1 50 13.5 100 5 1,000 0 28 50 48 100 15 1,2500 29 100 20 1,500 50 94 2,000 50 92 2,500 0 50 50 65 100 21

The amount of dopant may also affect the mobility of the semiconductorfilm containing zinc, nitrogen, and oxygen. Tables XIII and XIV show themobility, sheet resistance, carrier concentration, and resistivity forvarious nitrogen and oxygen flow rates when reactively sputtering a zincsputtering target that is doped with aluminum. Thus, the amount ofdopant in the sputtering target may be tuned to ensure a predeterminedmobility, sheet resistance, carrier concentration, and resistivity areachieved.

TABLE XIII Per- Oxygen Nitrogen Mobil- Sheet Carrier cent Flow Flow ityResis- Concen- Resis- Dop- Rate Rate (cm²/ tance tration tivity ant(sccm) (sccm) V-s) (ohm-cm) (#/cc) (ohm-cm) 0.0 20 100 3 100 1.00E+210.008 200 13 500 8.00E+19 0.009 300 28 600 2.00E+19 0.015 400 42 8001.00E+19 0.020 500 45 950 9.00E+18 0.040 1.0 10 100 1 300 1.00E+21 0.009200 14 600 5.00E+19 0.020 300 31 975 9.00E+18 0.040 400 33 2500 5.00E+180.080 500 33 5000 3.00E+18 0.090 1.0 20 100 14 800 8.00E+19 0.020 200 275000 4.00E+18 0.100 300 29.0 5,000 4.00E+18 0.100 400 29.0 9,0002.00E+18 0.100 500 33.0 9,000 1.50E+18 0.200

TABLE XIV Per- Nitrogen Oxygen Mobil- Sheet Carrier cent Flow Flow ityResis- Concen- Resis- Dop- Rate Rate (cm²/ tance tration tivity ant(sccm) (sccm) V-s) (ohm-cm) (#/cc) (ohm-cm) 0.0 300 0 0 100 1.00E+210.009 10 4 200 2.00E+20 0.009 20 30 500 2.00E+19 0.010 30 43 8001.00E+19 0.020 40 9 4,000,000 1.00E+19 8.000 1.0 300 0 0 700 3.00E+210.030 10 31 950 9.00E+18 0.020 20 31 1,000 2.00E+18 0.040 30 23 1,2009.00E+17 0.100 40 15 11,000 9.00E+17 0.700 1.0 500 0 2 8,000 2.00E+190.200 10 34 7,500 5.00E+18 0.090 20 33 9,000 1.50E+18 0.100 30 25 13,0005.00E+17 0.700 40 15 13,000 5.00E+17 0.700

The temperature of the susceptor may also influence the mobility of thesemiconductor film. Table XV shows the mobility, sheet resistance,carrier concentration, and resistivity for various nitrogen flow ratesin sputtering a zinc sputtering target at temperatures of 30 degreesCelsius, 50 degrees Celsius, and 95 degrees Celsius. As may be seen fromTable XV, the reactive sputtering may effectively form a semiconductorfilm having mobility higher than amorphous silicon and polysilicon attemperatures significantly below 400 degrees Celsius, includingtemperatures approaching room temperature. Thus, even without annealing,the semiconductor film may have a higher mobility than amorphoussilicon.

TABLE XV Susceptor Sheet Carrier Nitrogen Temper- Resis- Concen- Resis-Flow Rate ature Mobility tance tration tivity (sccm/m³) (Celsius)(cm²/V-s) (ohm-cm) (#/cc) (ohm-cm) 500 30 1.0 200 1.00E+21 0.009 50 1.5210 2.00E+19 0.008 95 2.0 300 4.00E+18 0.014 1,500 30 15.0 1,1001.00E+21 0.030 50 31.0 950 9.00E+18 0.029 95 17.0 850 4.00E+18 0.0282,500 30 28.0 3,100 7.00E+20 0.900 50 33.0 3,100 2.00E+19 0.078 95 32.02,950 4.00E+18 0.077

While the power may be described herein as specific values, it is to beunderstood that the power applied to the sputtering target isproportional to the area of the target. Hence, power values betweenabout 10 W/cm² to about 100 W/cm² will generally achieve the desiredresults. Table XVI shows the affect of the applied DC power on themobility, carrier concentration, and resistivity for nitrogen gas flowsof 1,500 sccm/m³ and 2,500 sccm/m³. Power levels between about 1,000 Wand 2,000 W produce semiconductor films having a mobility significantlyhigher than amorphous silicon.

TABLE XVI Carrier Nitrogen Concen- Flow Rate Power Mobility trationResistivity (sccm/m³) (W) (cm²/V-s) (#/cc) (ohm-cm) 1,500 1,000 345.00E+17 0.80 1,500 41 3.10E+18 0.08 2,000 31 7.20E+18 0.05 2,500 1,00030 4.00E+17 2.00 1,500 39 1.50E+18 0.10 2,000 34 2.50E+18 0.09

The film deposited according to the above discussed depositiontechniques may comprise a ternary compound semiconductor material havingzinc, nitrogen, and oxygen such as ZnN_(x)O_(y). In one embodiment, theternary compound semiconductor material may be doped such asZnN_(x)O_(y):Al. The ternary semiconductor compound may have a highmobility and a low electron carrier density when deposited at roomtemperature in contrast to zinc oxide which has a high electron mobilityand a high electron carrier density. In one embodiment, the ternarycompound has a mobility higher than 30 cm²/V-cm and an electron carrierdensity lower than 1.0e+19 #/cc. When the film is annealed at about 400degrees Celsius, the mobility may be increased to greater than 100cm²/V-cm and the electron carrier density may be lower than 1.0e+18 #/ccwithout changing the film crystallographic orientation and composition.The high mobility and low electron density may be achieved for theternary compound even when the film is an amorphous compound or poorlyoriented crystallographic compound.

The optical band gap of the ternary compound may also be improvedcompared to zinc oxide. Zinc oxide typically has a band gap of about 3.2eV. The ternary compound comprising zinc, nitrogen, and oxygen, on theother hand, may have a band gap from about 3.1 eV to about 1.2 eV. Theband gap may be adjusted by altering the deposition parameters such asnitrogen to oxygen flow ratio, power density, pressure, annealing, anddeposition temperature. Due to the lower band gap, the ternary compoundmay be useful for photovoltaic devices and other electronic devices. Atvery high processing temperatures such as 600 degrees Celsius, theternary film may be converted to p-type or n-type semiconductormaterial. The annealing or plasma treatment may be fine tuned withoutfundamentally changing the compound structure and chemical composition.The fine tuning permits the properties of the compound to be tailored tomeet the performance requirements of devices in which the compound maybe used.

The ternary compound may be useful as a transparent semiconductor layerin a TFT device, a compound layer in a photovoltaic device or solarpanel, or as a compound layer in a sensor device. FIGS. 4A-4G show aprocess sequence for forming a bottom gate TFT 400 according to oneembodiment of the invention. The TFT may comprise a substrate 402. Inone embodiment, the substrate 402 may comprise glass. In anotherembodiment, the substrate 402 may comprise a polymer. In anotherembodiment, the substrate may comprise plastic. In still anotherembodiment, the substrate may comprise metal.

Over the substrate, a gate electrode 404 may be formed. The gateelectrode 404 may comprise an electrically conductive layer thatcontrols the movement of charge carriers within the TFT. The gateelectrode 404 may comprise a metal such as aluminum, tungsten, chromium,tantalum, or combinations thereof. The gate electrode 404 may be formedusing conventional deposition techniques including sputtering,lithography, and etching. Over the gate electrode 404, a gate dielectriclayer 406 may be deposited. The gate dielectric layer 406 may comprisesilicon dioxide, silicon oxynitride, silicon nitride, or combinationsthereof. The gate dielectric layer 406 may be deposited by well knowndeposition techniques including plasma enhanced chemical vapordeposition (PECVD).

Over the gate dielectric layer 406, the active channel 408 (i.e.,semiconductor layer) may be formed as shown in FIG. 4B. In oneembodiment, the active channel 408 is annealed. In another embodiment,the active channel 408 is exposed to a plasma treatment. The annealingand/or plasma treatment may increase the mobility of the active channel408. The active channel 408 may comprise the ternary compound havingzinc, oxygen, and nitrogen as discussed above. In one embodiment, theternary compound is doped with aluminum. Once the active channel 408 hasbeen deposited, the source-drain layer 410 may be deposited over theactive channel 408 as shown in FIG. 4C. In one embodiment, thesource-drain layer 410 may comprise a metal such as aluminum, tungsten,molybdenum, chromium, tantalum, and combinations thereof. In order todefine the active channel 408 and the source-drain electrodes, both thesource-drain layer 410 and the active channel 408 may be etched.

As shown in FIG. 4D, a mask 412 may be disposed on the source-drainlayer 410. The mask 412 may have a predetermined pattern. The mask 412may be disposed on the source-drain layer 410 by conventional techniquesincluding photoresist deposition followed by pattern development.

The active channel 408 and the source-drain layer 410 may besimultaneously etched as shown in FIG. 4E. FIG. 4F shows a top view ofFIG. 4E. As can be seen in FIGS. 4E and 4F, the active channel 408 maybe exposed by the etching. In one embodiment, the active channel 408 maybe exposed by wet etching both the source-drain layer 410 and a portionof the active channel 408. In another embodiment, the source-drain layer410 may be dry etched followed by a wet etching of a portion of theactive channel 408. In another embodiment, the source-drain layer 410may be etched without etching the active channel 408. In one embodiment,the dry etching may be performed with a gas containing an elementselected from chlorine, oxygen, fluorine, or combinations thereof.

Following the exposure of the active channel 408, source-drainelectrodes may be defined by dry etching the source-drain layer 410using the active channel 408 as an etch stop layer. FIG. 4G shows a topview of the exposed active channel 408 and defined source electrode 414and drain electrode 416. The active channel 408 may function as an etchstop layer during dry plasma etching because the ternary compoundcomprising zinc, oxygen, and nitrogen (and in certain embodiments,aluminum) may not be effectively etched by plasma.

FIG. 5 is a schematic cross sectional view of an etch stop TFT 500according to one embodiment of the invention. The etch stop TFT maycomprise a substrate 502, a gate electrode 504, and a gate dielectriclayer 506. The etch stop TFT 500 is similar to the bottom gate TFT shownabove in FIGS. 4A-4G, but an etch stop 510 may be present over theactive channel 508 between the source electrode 512 and the drainelectrode 514. The materials for the substrate 502, gate electrode 504,gate dielectric 506, active channel 508, source electrode 512, and drainelectrode 514 may be as described above in relation to the bottom gateTFT. The etch stop 510 may comprise a dielectric material comprisingsilicon and one or more of oxygen and nitrogen.

FIG. 6 is a schematic cross sectional view of a top gate TFT 600according to one embodiment of the invention. The top gate TFT 600 maycomprise a substrate 602 having a light shielding layer 604 depositedthereon. A dielectric layer 606 may be deposited over the lightshielding layer 604. A source electrode 608 and a drain electrode 610may be deposited over the dielectric layer 606. The active channel layer612 may be deposited over the source electrode 608 and the drainelectrode 610. A gate dielectric layer 614 may be deposited over theactive channel 612 and the gate electrode 616 may be deposited over thegate dielectric layer 614. The materials for the substrate 602, gateelectrode 616, gate dielectric 614, active channel 612, source electrode608, and drain electrode 610 may be as described above in relation tothe bottom gate TFT. In forming the top gate TFT 600, the channel andelectrode contact area may be formed by wet etching or dry etchingfollowed by wet etching. Then, the contact area may be defined by dryetching using the active channel as an etch stop layer.

FIG. 7 is a schematic view of an active-matrix LCD 700 according to oneembodiment of the invention. FIG. 7 shows a TFT substrate and a colorfilter substrate with a liquid crystal material sandwiched therebetween.The TFT controls a current to the pixel electrode creating an electricfield to control the orientation of the liquid crystal material and thusthe amount of light that is transmitted through the color filter. TheTFTs are arranged in a matrix on a glass substrate. To address aparticular pixel, the proper row is switched on, and then a charge issent down the correct column. Since all of the other rows that thecolumn intersects are turned off, only the capacitor at the designatedpixel receives a charge. The capacitor is able to hold the charge untilthe next refresh cycle. If the amount of voltage supplied to a crystalis controlled, the crystal may untwist only enough to allow some lightthrough.

The active matrix LCD 700 shown in FIG. 7 may be controlled by a TFT714. The TFT 714 may turn pixels on or off. The LCD 700 may comprise abonding pad 706, a pixel electrode 708, a storage capacitor 728, apolarizer 702, a substrate 704, an alignment layer 710, a spacer 712, ashort 716, a seal 718, a black matrix 720, a color filter 724, and acommon electrode 724.

FIG. 8 is a schematic view of an active-matrix OLED 800 according to oneembodiment of the invention. FIG. 8 shows a TFT controlling the amountof current applied to the emissive organic layers of the OLED.Active-matrix OLEDs have full layers of cathode, organic molecules andanode, but the anode layer overlays a TFT array that forms a matrix. TheTFT array itself is the circuitry that determines which pixels getturned on to form an image. Active-matrix OLEDs consume less power thanpassive-matrix OLEDs because the TFT array requires less power thanexternal circuitry, so they are efficient for large displays.Active-matrix OLEDs also have faster refresh rates suitable for video.Active-matrix OLEDs may be used in computer monitors, large screen TVs,electronic signs, and electronic billboards.

The OLED 800 may be controlled by a TFT 802 that turns the pixels on oroff. The OLED 800 comprises a TFT 802, an anode 804, a hole injectionlayer 806, an ionization layer 808, a buffer layer 810, a transparentcathode 812, an emissive layer 814, and an encapsulation layer 816.

FIGS. 9A-9C show the Vth for various active channel lengths and widths.FIG. 9A shows the Vth for a active channel length of 40 μm and a widthof 10 μm. FIG. 9B shows the Vth for a active channel length of 80 μm anda width of 10 μm. FIG. 9C shows the Vth for a active channel length of80 μm and a width of 20 μm. In each of FIGS. 9A-9C, it is shown that theternary compound active layer has a high on-off ratio and a highcurrent.

FIGS. 10A-10C show a comparision of the Vth for active channels having acommon length and width. The active channel width is 40 μm and the widthis 10 μm. FIG. 10A is the Vth for amorphous silicon. FIG. 10B is theternary compound without annealing. FIG. 10C is the ternary compoundafter annealing. The ternary compound TFT of FIG. 10B has a drain-sourcecurrent at Vg=1 that is higher than the drain-source current foramorphous silicon at 10 V. Therefore, the non-annealed film is about 10times better than amorphous silicon.

The annealed film in FIG. 10C is even better. The annealed film has ahigh saturation current as compared to the non-annealed film. Thesource-drain current for the annealed film at Vd=0.1 V is close to theamorphous silicon TFT's current at Vd=10V. Thus, the annealed film isabout 100 times better than the amorphous silicon.

The performance of the TFTs described above can be adjusted or tuned bychanging the active layer carrier concentration, the active layermobility, and the characteristics of the active layer at the interfacewith other layers. The TFTs may be tuned by changing the nitrogencontaining gas flow rate during the film deposition. As noted above, theratio of the nitrogen containing gas to the oxygen containing gas usedin sputter deposition of the active layer may affect the mobility,carrier concentration, and other factors. A user may set a predeterminedvalue for the carrier concentration, the mobility, or othercharacteristic and then adjust the nitrogen to oxygen flow ratioaccordingly to produce the desired film properties. The adjustment mayeven occur in response to an in situ measurement to permit real timecontrol of the deposition process.

The TFTs may also be tuned by the amount of aluminum doping. The amountof aluminum doping may be predetermined so that a sputtering target ofappropriate composition may be disposed within the processing chamber.Additionally, the TFTs may be tuned by annealing and/or plasma treatingthe active layer after deposition or during integration with otherfilms. As noted above, heat treating the ternary compound may increasethe mobility of the film.

TFTs comprising oxygen, nitrogen, and one or more elements selected fromthe group consisting of zinc, tin, gallium, indium, and cadmium haveincreased mobility over TFTs made with amorphous silicon. The increasedmobility of the TFTs permits the TFTs to be used not only for LCDs, butalso for the next generation of displays, OLEDs.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A thin film transistor, comprising: a semiconductor in the transistorlayer comprising a compound selected from the group consisting of: anoxynitride compound comprising oxygen, nitrogen, zinc, indium andgallium; an oxynitride compound comprising oxygen, nitrogen, zinc andtin; and a compound comprising oxygen, zinc, indium and gallium.
 2. Thetransistor of claim 1, wherein the transistor is a top gate thin filmtransistor.
 3. The transistor of claim 1, further comprising: asubstrate; a gate electrode disposed over the substrate; a gatedielectric layer disposed over the gate electrode; the semiconductorlayer disposed over the gate dielectric layer; and source and drainelectrodes disposed over the semiconductor layer and spaced apart todefine an active channel.
 4. The transistor of claim 3, furthercomprising an etch stop layer disposed over the semiconductor layer inthe active channel.
 5. The transistor of claim 1, wherein thesemiconductor layer has a mobility of greater than about 50 cm²/V-s. 6.The transistor of claim 1, wherein the compound further comprises adopant selected from the group consisting of Al, Ca, Si, Ti, Cu, Ge, Ni,Mn, Cr, V, Mg, and combinations thereof.
 7. The transistor of claim 6,wherein the compound comprises aluminum dopant.
 8. The transistor ofclaim 1, wherein the compound comprises oxygen, zinc, gallium, andindium.
 9. The transistor of claim 8, wherein the compound comprisesaluminum dopant.
 10. The transistor of claim 1, wherein the compoundcomprises an oxynitride compound comprising oxygen, nitrogen, zinc,indium and gallium.
 11. The transistor of claim 10, wherein the compoundfurther comprises a dopant.
 12. The transistor of claim 11, wherein thedopant comprises aluminum.
 13. The transistor of claim 1, wherein thecompound comprises an oxynitride compound comprising oxygen, nitrogen,zinc and tin.
 14. The transistor of claim 13, wherein the compoundfurther comprises a dopant.
 15. The transistor of claim 14, wherein thedopant comprises aluminum.